The invention relates to a speed-difference-dependent hydraulic clutch which comprises a drive housing, which rotates in a housing that is fixed in space and forms a reservoir for a working fluid, a first shaft which can be connected in a rotationally fixed manner to the drive housing, a second shaft which is connected in a rotationally fixed manner to the drive housing, and furthermore a hydrostatic displacement machine, a friction clutch for connecting the first shaft to the drive housing and a piston for acting on the friction clutch, which are accommodated in the drive housing, the drive housing having a pressure space, and where a pressure is produced in the pressure space when a difference occurs between the speed of the drive housing and that of the second shaft, this pressure giving rise to action upon the friction clutch, and a passage leading from the pressure space to a control valve which opens up the route to a space of lower pressure in a controlled manner.
Clutches of this type are used in various configurations in the drive train of motor vehicles for direct transmission of a torque or for locking a link differential for driving the wheels of one axle or for distributing the drive torque between two axles. The hydrostatic displacement machine comprises an inner rotor and an outer rotor, for example, but is equally possible to use hydrostatic displacement machines of some other kind. In either case, two parts (for example, the drive housing and the output shaft) perform a relative movement when there is a speed difference and thus produce a pressure that acts on the piston of the friction clutch.
A speed-difference-dependent hydraulic clutch of this type is disclosed in U.S. Pat. No. 4,924,989. That part of the shaft, which can be connected to the drive housing, that protrudes into the clutch is design there as a control valve which contains a control element producing the connection between the delivery and intake sides of the pump. On account of the control valve being situated in the connectable shaft, two rotary inputs are required for connecting it to the delivery and intake sides of the pump. A third rotary input is used to feed the fluid, which is placed under pressure by the brake cylinder during braking, to the control valve in order to actuate the control element. This renders the control valve inaccessible to such an extent that it can only be actuated hydraulically. However, this is not expedient in a genuine control system which is not coupled to the brake system.
A further speed-difference-dependent hydraulic clutch of this type is disclosed in U.S. Pat. No. 5,358,455. In this, the control valve is arranged in the drive housing, on the outer circumference thereof, and is actuated by an actuator fixed on the housing via a selector ring which surrounds the drive housing. As a result, the control element of the control valve is exposed to the maximum centrifugal force and the selector ring cannot be precisely actuated due to its large diameter; overall, this is not a practicable solution.
It is therefore the object of the invention to design a clutch of the generic type in such a manner that sensitive, precise and rapid control and adaptation of the characteristic is ensured, with an outlay which is as small as possible.
According to the invention, this is achieved by virtue of the fact that the control valve is arranged in the drive housing and/or, in the second shaft, which is connected in a rotationally fixed manner to the drive housing, is arranged coaxially with said shaft, and has a coaxial control element which can be displaced in the direction of the axis and is actuated by an actuator which is fixed on the housing. The control valve therefore always rotates together with the control element, which means that rotary inputs are no longer required, and, owing to its coaxial arrangement, it is not subjected to centrifugal force. Since the drive housing and first shaft are connected fixedly to each other or are even a single part, the control valve is accommodated in the drive housing at a point to which the passage can be directly brought; and yet the actuation can take place from the second shaft, i.e. by means of a selector element of smaller diameter which is guided with greater precision and less friction.
In a development of the invention, the control valve comprises the control element and a valve body having a cylindrical surface which is coaxial with the axis of rotation and is intended for the control element, the coaxial cylindrical surface being open into the interior of the drive housing and the passage leading from the pressure space into an inlet opening on the cylindrical surface. The control element is accordingly a coaxial control piston and the fluid flow released by it can flow into the interior of the friction clutch where it can cool and lubricate it.
A particularly simple construction and reliable functioning is obtained by virtue of the fact that the control element is a cylindrical pin which is loaded in the opening direction of the valve by a compression spring. A pin of this type can be produced with exacting tolerance and the compression spring ensures that the clutch is very rapidly released if there is a defect.
The control valve is a proportional valve which is controlled as a function of operating states. This enables the torque which is transmitted by the clutch to be controlled electronically as a function of driving states and, if appropriate, the driver's wishes.
In a further development of the basic concept of the invention, an actuator which is arranged outside the second shaft, which is connected on a rotationally fixed manner to the housing, is fixed on the housing and acts on the control element is provided. This enables the rotating control element to be actuated from the stationary housing.
In a first embodiment, the actuator interacts with a selector sleeve which surrounds the second shaft, rotates together with it and can be displaced in the axial direction, and a rod which passes through the selector sleeve and the second shaft and comes into contact with the control element is provided, the second shaft having a slot for the rod to pass through. Since the selector sleeve sits on the shaft, its diameter can be small. The rod permits the movement into the interior of the shaft to be introduced and is guided precisely in the slot.
In a particularly advantageous embodiment, the actuator is a solenoid which surrounds the second shaft and acts on the control element through a magnetically neutral region of the second shaft. This enables the field of the solenoid, which is fixed on the housing, to act on the control valve through the shaft without any contact and without losses due to friction. In this case, it is particularly favorable for the valve body to be inserted into a central hole of the drive housing and for the control element to be a pin with a magnet core.